Seismic Performance Estimation For Skew Bridges Subjected To Horizontal-and Vertical-Near-Fault Ground Motions

The influence of coupled ground motions on the performance of structures in near-fault regions still not clear for special structures with different characteristics of ground motions and site conditions.A few available studies in the literature that explored the impacts of SSI and near-fault ground motions on the seismic response of skewed bridges,which could induce damage to the bridge structures.The main focus of this research is to investigate the influence of single and coupled(horizontal and vertical)near-fault ground motions on the structural performance of skewed RC isolated-bridge with considering the soil-structure interaction(SSI)effects.Three-span Continuous skewed RC bridge taken from Federal Highway Administration(FHWA,1996)design example No.4 is used for this investigation.The bridge structure is modeled as a 3D model utilizing the finite element computer software SAP2000.The nonlinear time history analysis is carried out to investigate the dynamic response of isolated bridges under single and coupled(vertical and horizontal)of seven ground motions with various intensities.Five records characterized as nearfault records with a pulse(NFP)and two records as near-fault without a pulse(NFNP).The triple friction pendulum(TFP)bearing systems is employed to isolate the superstructure of the bridge.The mechanical properties and geometric dimensions needed for simulating(TFP)in the used computer software is designed and calculated to capture the actual behavior of such isolation devices.The effectiveness of this bearing in such conditions is addressed comparing the response of the isolated and nonisolated bridge.Besides,the influence of soil-structure interaction is also investigated considering two kinds of site conditions classified as soil C and D,according to AASHTO.The soil is modeled as springs in the base of the piers and abutments with six degrees of freedom utilizing the spring approach.The comparison is made in piers and girder demands in three parts: when the bridge subjected to horizontal and coupled(vertical and horizontal),the isolated with the non-isolated bridge,and including/excluding SSI.Form the results;It concluded that the axial forces of piers are considerably increased in the case of coupled ground motions compared with the horizontal one.In contrast,coupled effects are negligible on the girder’s demands.TFPs have beneficial effects on the pier demands for all cases.In contact,they have detrimental effects on the bending moments and shear forces of the piers when the bridge is exposed to longitudinal excitations(L)and combined longitudinal and vertical excitations(L+V).In addition,they have detrimental effects on the bending moments and shear forces of the girder for all cases.Considering SSI,the pier and girder demands tend to increase with including SSI and with increasing the flexibility of soil.